Recently, a sheet finishing apparatus is provided after an image forming apparatus (for example, MFP) in order to carry out finishing of sheets after image formation. The sheet finishing apparatus is called finisher. The finisher punches holes in or staples sheets sent from the MFP and sorts and discharges the sheets.
The finisher has a lateral alignment board which controls the position of sheets in the direction of width. The finisher aligns the sheets in the direction of width, carries the sheets to a stapler, and staples the sheets. The lateral alignment board is also used to sort and discharge sheets. In punching a hole in sheets by using a punch unit, the skew quantity of sheets carried from the image forming apparatus is measured and a hole is punched in the sheets inclined in accordance with the skew quantity.
In the conventional finisher, when a user sets sheets in a cassette, misalignment of the sheets may occur and cause an error, or misalignment of the sheets may occur in the direction of width of the sheets at the time of image formation. Moreover, very high temperatures in forming an image on sheets may cause sheets to expand or contract and the sheets may cause an error in the direction of width. As the misalignment (error) in the direction of width of the sheets increases, the sheets collide with a tray or the like in the finisher while the sheets are carried, and cause a jam. There is also a problem that sheets fall on the lateral alignment board and cannot be aligned correctly.
The punch unit is provided with a lateral edge detection sensor which detects the position of sheets in the direction of width, and a skew sensor which measures the skew quantity of sheets. However, the lateral edge detection sensor and the skew sensor are only used in the punch unit.
JP-A-2006-273543 discloses an image forming system having an alignment unit which aligns sheets in the direction of width. JP-A-2007-193193 discloses an image forming apparatus having a detection unit which detects a sheet edge on a carrying path.